1) Field of the Invention
The present invention relates to a plasma torch, more particularly, to a plasma torch that has high efficiency, the input power of which is more than 1 megawatt (MW), and the cathode of which has long life span.
2) Background of the Invention
in "UIE ARC PLASMA REVIEW 1998" which illustrates many kinds of plasma torches, among them two representative plasma torches will be explained.
FIG. 2 shows one of the transferred-solid type plasma torches of the prior art, a Daido Steel brand plasma torch. This is a scaled-up version of the torch design used for cutting and welding. The long tungsten cathode is recessed behind the copper nozzle and operates in the transferred mode with argon gas. The torch is available in sizes up to about 1 MW
FIG. 3 shows one of the non-transferred hollow type plasma torches of the prior art, an SKF brand, a SKF plasma torch. This design is segmented with a fixed length arc column with magnetic field coils at each end. The torch has two equal diameter copper electrodes, the capped rear electrode being connected negative. So the magnetic field makes a rotating arc root in the ups electrode and this results in long cathode life. The insulated segments between the electrodes stretch the length of the arc column to a larger length that will develop a higher arc voltage. Torches with power ratings from 100 kW to 8 MW are available. The SKF torch has a field coil for rotating the arc foot of the upstream electrode and thus can make more than 1 MW output. However efficiency of the torch is lower than the transferred type since temperature of the plasma rapidly reduces as it departs from the nozzle. To exchange the cathode, the whole assembly surrounding the cathode must be removed, which results in high exchange cost.